Field of the Invention
The present invention relates to a solid-state imaging device and an endoscope system.
Description of Related Art
A technology which enables observation of blood vessels in a mucosal surface layer using narrowband light having a wavelength of about 410 nm has been disclosed as shown in Japanese Unexamined Patent Application, First Publication No. 2012-170639. It is possible to obtain images in which blood vessels are emphasized by irradiating blood vessels with narrowband light which is easily absorbed by hemoglobin in the blood. Three-color images and narrowband-light images are obtained, for example, using a solid-state imaging device (an image sensor) in which pixels corresponding to visible light of three colors, red (R), green (G), and blue (B), are arranged.
The arrangement of color filters in pixels in the solid-state imaging device which can obtain three-color images is, for example, a Bayer pattern. FIG. 14 shows a unit array F10 of color filters constituting a Bayer pattern. As shown in FIG. 14, the unit array F10 has one red color filter Fr10, two green color filters Fg10, and one blue color filter Fb10. The unit array F10 shown in FIG. 14 is arranged two-dimensionally in the solid-state imaging device.
FIG. 15 shows the spectral transmission characteristics of color filters of three colors. In the graphs shown in FIG. 15, the horizontal axis represents the wavelength and the vertical axis represents the transmittance of the color filters. The graph Gr20 shows the spectral transmission characteristics of the red color filter. The graph Gg20 shows the spectral transmission characteristics of the green color filter. The graph Gb20 shows the spectral transmission characteristics of the blue color filter. The red color filter has a transmittance peak PKr20 at a wavelength near 610 nm corresponding to a red wavelength. The green color filter has a transmittance peak PKg20 at a wavelength near 540 nm corresponding to a green wavelength. The blue color filter has a transmittance peak PKb20 at a wavelength near 450 nm corresponding to a blue wavelength.
The transmittance of the blue color filter is high at 410 nm which is a wavelength of narrowband light. On the other hand, the transmittance of each of the red and green color filters is low at 410 nm. Therefore, narrowband light is likely to be detected only at pixels having blue color filters and is unlikely to be detected at pixels having red and green color filters.
The resolution of narrowband light images is low because narrowband light is unlikely to be detected at pixels having red and green color filters. In order to improve the resolution of narrowband light images, it is desirable that each of the red and green color filters have transparency to the wavelength of narrowband light. However, when a subject is irradiated with white light, red, green, and narrowband light are detected at pixels, thereby lowering the color resolution of color images. Therefore, it is conceivable to use red color filters having a steep peak transmittance at the wavelength of narrowband light. Similarly, it is conceivable to use green color filters having a steep peak transmittance at the wavelength of narrowband light.
FIG. 16 shows the spectral transmission characteristics of a green color filter having a steep peak transmittance at the wavelength of narrowband light. In the graph Gg21 shown in FIG. 16, the horizontal axis represents the wavelength and the vertical axis represents the transmittance of the color filter. The color filter has a transmittance peak PKg21 at a wavelength near 540 nm corresponding to a green wavelength. The color filter also has a transmittance peak PKg22 at a wavelength near 410 nm corresponding to the wavelength of narrowband light.
When a subject is irradiated with narrowband light, the narrowband light passes through the color filter due to the spectral transmission characteristics thereof near the peak PKg22. Therefore, the pixel can detect the narrowband light. On the other hand, when the subject is irradiated with white light, green light passes through the color filter due to the spectral transmission characteristics thereof near the peak PKg21. Therefore, the pixel can detect the green light. When the subject is irradiated with white light, narrowband light also passes through the color filter. Deterioration of the color resolution of color images is suppressed since a range of wavelengths having transparency near the peak PKg22 is narrow.